Novel isothiocyanates of thiocolchicine and colchicine allocongeners were prepared as potential markers of the colchicine binding site on tubulin. The most interesting compound is 9-isothiocyanato-9-deoxy-N-acetylcolchinol which will be prepared in radiolabeled form to ensure specificity. Analogs of the potent inhibitor N-acetylcolchinyl methyl ether lacking methoxy groups at C(1), or at C(9), were found to be not very active. Several compounds of the colchicine series which showed good in vitro activity against human tumor cell lines will be tested in vivo.